1. Technical Field
This invention relates generally to spark ignition devices, such as spark plugs for internal combustion engines, and more particularly to ground electrodes attached to a metal shell of the spark ignition device and to their method of attachment to the metal shell.
2. Related Art
Modern automotive vehicles are required to meet increased power, low fuel consumption, and low exhaust emissions requirements, thus resulting in an increase in temperature of burning atmosphere in the engine. Therefore, weld joints between a metal shell of a spark ignition device and a ground electrode are subjected to increased temperatures, and thus, have become more prone to cracking, thus resulting in separation of the ground electrode from the metal shell.
When the ground electrode is joined to the metal shell using typical laser welding techniques, a weld joint folioed between them is usually small, which could result in a lack of the strength of the joint. In addition, the laser weld joint process typically results in the material of the ground electrode and the metal shell expelling radially inwardly into a cavity of the shell and/or radially outwardly from the shell. As such, secondary, inefficient and costly manufacturing operations are needed to clean-up the expelled material. Further yet, if the laser weld joint is formed with a gap or voids existing and remaining between the ground electrode and the shell, the laser weld joint can be subject to premature failure.
In accordance with other known processes, the ground electrode can be resistance welded to the shell. However, a weld joint formed solely by a resistance weld process generally requires the ground electrode to be upset, i.e. pushed into the material of the shell while high current flows, thereby causing material of the ground electrode and the metal shell to be expelled as discussed above, thus requiring secondary, inefficient and costly manufacturing operations to clean-up the expelled material. Further, a resistance weld joint is formed primarily as a “forge weld,” which produces limited fused material in the weld joint, thus lending to a weld joint that has relatively low strength.
In addition to the problems associated with the known processes discussed above, accurately positioning the ground electrode relative to the metal shell and providing an improved heat transfer path between the ground electrode and the shell remain an area where advances are sought for improvement.
A spark ignition device constructed in accordance with this invention addresses these and other issues, as will be apparent to one having ordinary skill in the art.